Thermal batteries using molten nitrate electrolytes offer significantly higher cell voltages and marked improvements in energy and power densities over present thermal batteries. However, a major problem is gas-evolution reactions involving the molten nitrate electrolytes. This gassing problem has blocked the advantages offered by thermal batteries using molten nitrates. The solution to this gassing problem is to eliminate the chloride ion contaminates. The most important step in reducing chloride contamination is the avoidance of potassium perchlorate (KClO.sub.4) or any other chlorine-containing substances that can decompose to produce chloride ions in any thermal battery component. The Fe+KClO.sub.4 pyrotechnic used to activate thermal batteries is a key example. The decomposition of such substances into chloride ions (Cl--) results in passivating-film breakdown and gas-producing reactions with the molten nitrate electrolyte. These reactions largely involve the lithium-component of the anode used in thermal batteries such as Li--Fe (LAN), Li--Si, and Li--Al. The introduction of chloride ions into the nitrate melt via the pyrotechnic materials produces a rapid breakdown of the protective oxide film on lithium-based anodes and leads to gas-producing reactions.